A variety of applications exist for devices capable of switching high electric currents in electric power distribution systems. A common application in the United States and Canada for high- current switches is referred to as "load-break" service. In this application, the switch controls transmission of power from a supply circuit to a load circuit. Several load break switches, each having a separate load circuit associated therewith, may be connected in parallel to a single supply circuit in order to control power distribution to the several load circuits. A load circuit typically may receive power from one or two supply circuits, and would have a load break switch for controlling each of the connections between the load circuit and the supply circuits. The primary function of a load break switch is to control power distribution to loads, and although a load break switch may be capable of switching fault currents, fault handling is not the function of a load break switch. Some switches in load break service are operated relatively frequently.
Another common application for a high-current switch is "circuit breaker" service, in which the function of the switch is to control power during a fault condition. Such switches usually are capable of handling a fault current which far exceeds the normal operating current of the switch. Because circuit breaker switches are primarily used to interrupt fault currents, they may be operated very infrequently.
An application common in Europe for high-current switches is "ring-main" service. In that application, a load circuit is organized as a "ring" with power applied to the ring from a single supply circuit at a single place. Subsidiary load circuits are connected to the main load circuit at various points around the ring. Because the main load circuit is arranged as a ring, power can flow from the supply circuit to a subsidiary load in either direction along the ring, thereby minimizing the effect of an interruption at any point along the ring. Several ring main switches are inserted serially at various points along the ring to control the flow of power past those points. By opening any two switches in the ring, the segment of the ring between the switches may be isolated from the supply circuit without affecting loads on either side of the isolated segment. Switches used in ring main service generally have electrical characteristics similar to those of load break switches. Ring main configurations generally require a relatively large number of switches, in order to permit individual subsidiary load to be isolated.
Regardless of application, most high-current switches are subject to arcing and its attendant deleterious effects. Arcing can cause the contacts to erode and perhaps to disintegrate over time. In some atmospheres, the arc might cause an explosion. Therefore, a known practice is to fill the device with an inert, electrically insulating gas, such as sulphur hexafluoride (SF.sub.6), which quenches the arcing.
In order to most efficiently quench the arc, it is often desirable to direct the insulating gas toward the region where the arc may form, and to increase the pressure of the gas. Directing the gas toward the arc zone at high velocity improves quenching by physically disrupting the conductive path formed by hot ionized particles which result from the arc. Increasing the gas pressure improves quenching by increasing the ionization potential of the gas. Providing a high-pressure, high-velocity stream of gas may be accomplished several ways. The stream may be supplied from an external high-pressure reservoir. Alternatively, means may be provided inside the switch to direct and compress an existing supply of gas as a function of the movement of the switch contacts themselves. Switches providing such means are commonly referred to as "puffer" switches. As the switch moves its contacts in an arc-causing motion, the gas is compressed. A jet or nozzle is positioned so that at the proper moment during contact movement, when the arc might be forming, a draft or blast of the compressed gas is directed toward the area of the arc, in effect "blowing out" the "flame" of the arc.
A variety of puffer switches are known including the following U.S. Pat. Nos.: 2,757,261; 3,214,550; 3,749,869; 3,947,650; 4,268,890; 4,484,047; 4,490,594; 4,523,235; 4,527,029; 4,659,886; European Patent Nos.: 0,171,352; 0,214,083; West German Patent Nos.: 1,290,223; 2,333,895; PCT application No. 89/11746; and other devices: Merlin Gerin Fluarc FB; Siemens 8DJ10 Ring Main Units. However, various structural problems are common in such prior art puffer switches.
One problem with the prior art puffer switches is that the drive system does not effectively translate the rotating energy of the operating mechanism to linear motion to move the switch contacts. A typical prior-art puffer switch includes one or more corresponding pairs of electrical contacts located in an enclosure having an atmosphere of insulating gas. One of each pair of contacts is stationary. The remaining "moving" contact is mounted for substantially linear translation between a "closed" position, in which it mechanically and electrically engages the stationary contact, and an "open" position, spaced a substantial distance from the stationary contact to prevent current flow between the contacts. Each pair of contacts is located within a cylindrical chamber for partially confining the insulating gas. An angularly rotatable actuator arm is provided to permit a user to operate the switch. The actuator arm drives an operating mechanism. The operating mechanism, in turn, drives an axially rotatable drive shaft, which is often cylindrically shaped. Operating levers mounted on the drive shaft convert the rotational motion of the drive shaft to linear motion for driving the moving switch contacts. Often, a separate pair of operating levers would be provided for each movable contact and the levers would be located at longitudinal positions along the drive shaft corresponding to the locations of the movable contacts. Proper rotational behavior requires that the operating levers do not slip or break from the drive shaft. Prior art operating levers mounted on a cylindrical shaft were subject to slippage when the shaft was axially rotated to open or close the contact switches. Additionally, when metal set screws were used in the prior art to mount the operating levers onto the cylindrical shaft, the set screws were subject to breakage.
Another problem with the prior art puffer switches was that the operating levers and the set screws holding the contacts were made from conductive materials, such as metal. Thus, because the metallic components were typically at ground potential and the switch contacts were at high potentials, there was the possibility for an arc to appear between the contacts and the operating levers or set screws. Nonmetallic materials, such as plastic, however, were often considered inadequate for attaching the operating levers to the cylindrical shaft of prior art switches due to the force generated from the sudden axial movement of the shaft when the contact switches are closed and opened. Consequently, random arcing problems in prior art switches, metallic components and other similar conductors were utilized because they minimized rotational problems attributal to breaking and slipping of the operating levers on the cylindrical shaft.
Another problem with the prior art switches was longitudinal slippage and imperfect alignment of the operating levers along the cylindrical shaft. These problems were attributed to the cylindrical design of the shaft which made proper attachment of the levers difficult.
A further problem with the prior art switches was substantial stress concentrations at the connection between the operating mechanism and the drive shaft. This problem is further exacerbated where the operating mechanism is metallic. Typically, the operating mechanism had a rotating plate for securing the drive shaft positionally and for transferring the rotational energy of the operating mechanism to the drive shaft This plate often did no effectively transmit its rotating motion to the shaft. One aspect of this problem was that in order to secure the drive shaft to the rotating plate, the attachment means apply sufficient force to crush or otherwise damage the drive shaft. Even if the shaft were not crushed, during switch operation, the plate could apply sufficient forces to the shaft to cause the shaft to deform at the point of contact, producing rotational tolerance errors. In addition, the tight coupling used between the plate and the drive shaft was intolerant of longitudinal stresses or displacements of the drive shaft. Where loose coupling was used between the plate and the shaft, the impact of the rotating plate would cause damage to the shaft, and the loose coupling also introduced rotational tolerance errors.
A further problem with prior art puffer switches was the mechanical arrangement for supporting the stationary contacts and for confining a quantity of insulating gas to be used to create the "puffing" effect. In some prior art switches, for example, an insulating base casting or base plate was used to support each of the cylindrical gas-confining chambers. Each chamber, in turn, supported an end cap, which was used to mount the stationary contact in the chamber. A problem confronting the designers of such switches was how to securely attach the chambers to the base casting and to the end caps. It is important for proper operation of the switch that the stationary contacts be securely held in a predetermined fixed position within the chamber, to ensure that current flows through appropriate regions of the contacts and so that contact is made or broken at the desired time. In many switches, a tie-rod type fastener system was used to compress the cylindrical chamber between the end caps and the base casting. In order to provide sufficient mechanical stability for this assembly, the tie-rod was adjusted to provide strong compressive forces. Because of the large compressive loads on the cylindrical chambers, a strong and relatively expensive material, such as polysulfone, was required for construction. This increased production costs.